O-I Superheavy Tank and other Heavy Tank Projects


Until recently information on the O-I (Oh-Ee) tanks was elusive and difficult to obtain, even for tank / military “otaku” (fanatics) in Japan. There were original blueprints, but they were held in Japanese archives and not on public display. Even some Japanese tank “dictionaries” claiming to be comprehensive did not include more than perhaps a passing mention of the O-I project. Since the release of these blueprints in 2015, however, there is now little doubt that the Japanese had such a project for these “superheavy” tanks.  As with all new discoveries, this throws a bit of a wrench in well-established stereotypes of what Japanese tank designers were capable of, especially from those who feel their knowledge on such subjects is already complete and cannot be thrown on its head.


Blueprints of the O-I, Photo Credit: Asahi News

To the casual observer of Japanese tanks and fans of the armor of other countries, the inclusion of the O-I tanks in games where Japan had no such tanks represented before may be controversial, depending on how historically accurate the game intends to be. Clearly the fans of a comprehensive Japanese Tech Tree for fantasy 16 vs. 16 tank games, such as the one at http://japangroundforces.weebly.com/ would like to see a large, powerful tank exist for a country where such tanks are hard to find, or relegated to prototypes for the most part. Having a competitive tank tree that matches up with other nations, not to mention one that keeps pace with Japanese airplanes, is appealing in these games. In the interest of game balance between nations, some game developers may see fit to add the O-I tanks. World of Tanks did just that, and went one step beyond by adding different variations of the Japanese superheavy tanks. However, there is little doubt that the O-I tanks, along with several other prototypes and even production Japanese tanks, did not see combat. So the inclusion of these tanks in games should be limited to games that allow prototypes and so-called “paper tanks”  — or perhaps more specifically partially-finished prototypes, of which the O-I would qualify per the information available — on all sides.

Japan had experience building large tanks with multiple turrets and a large main gun dating back to 1927 when they developed the “Experimental Tank #1”.  This model was followed by two more variants which ultimately resulted years later in the final Type 95 Heavy Tank in 1935. While relatively unknown in the West, the tanks were on display for a time at the Yasukuni Shrine.  Their continued development was in part a result of anticipated conflicts with armor from the Soviet Union, but ultimately they proved unsuited for deployment in China and the Pacific. The O-I was much heavier and larger, with more and significantly thicker armor plating, but some similar design elements can be seen between the O-I and the older heavy projects that came before it.


1930’s model Heavy Tank prototype on display at the Yasukuni Shrine

What did (or would) the O-I “150t” look like?  Thanks to a release on December 2015 from FineMolds, a Japanese model manufacturer that purchased the rights to use the blueprints currently archived, we now have a good look at what the O-I could have looked like in production form.  Of course it is fair to point out that they never did go to production, and if they had, further changes and tweaks might have been made in the details. Certainly things like adding camouflage and images of the O-I in battle are taking some artistic license. One can imagine places for shovels, rolled up canvas, or other tools filling blanks spots on the slab-like surfaces if the O-I had made it to the field.

What more proof is there of the O-I’s existence as a project, beyond the dusty tomes and recently surfaced blueprints, and perhaps a piece of tank tread?  There is much speculation and translation of existing documentation that continues to be released, and some previous estimates, project names, and other details may prove to be wrong even as these are rushed into games.

The armament is speculated to have been a modified and no doubt updated version of the Type 92 105mm field cannon. It had a muzzle velocity of 765m/s. It was widely used by the Japanese dating back to its introduction in 1932. Maximum range was 18,200 meters. There were smaller turrets and potential for mounting rockets.

There are 3  “ton” versions discussed, a 100t, 120t, and 150t.  In fact, the 120t appears to be the same as the 150t.

Per Dan Kanemitsu:

There was only one project, but since no written records surfaced (until recently), people were confused regarding the program. The chassis alone weighted 96 tons while final completed vehicle with turrets and armaments were projected to be 150 tons. Since only the chassis was complete, some people referred to it as the 100t tank program and speculated that the 150t tank program was a seperate one. … At some point some people thought the total weight was going to be 120t instead of 150t, and that’s why that number came about. This is all based on information that was recenly unearthed and used by Finemolds.

I had a friend directly ask the president of Finemolds regarding the 100t/120t confusion that that was the answer that was given.

Per a now rather dated article from 2013:

Shigeo Otaka who was a Mitsubishi engineer involved in the 100 ton tank’s development. He stated that he gathered information from other people involved in the project but due to the passage of time and the secret nature of the military program, he was unable to collect all details. The description made only from collective memories of those involved however the main features can be described:

The project was under the supervision of Colonel Murata from the 4th Technical Research Head Quarters.  Colonel Iwakuro from the Department of War gave the order to build the monster. The order to build the 100 ton tank came immediately after the defeat against the Soviet Union in the Nomonhan incident in 1939. LWG gives a quote from a position and ranked officer that is the same as Colonel Iwakuro but does not gave the officer’s name. Probably still Iwakuro. Quote and picture of Iwakuro below.


“I want a huge tank built which can be used as a mobile pillbox in the wide open plains of Manchuria. Top secret.”

Here is another quote from LWG of an undisputable requirement from the Colonel.


“Make the dimensions twice that of today’s tanks.”

It is clear that there is still some clean up work that experts can dig into now that the blueprints and documentation are available as of 2015. Perhaps just as with some disambiguation needed with Japanese armored cars, we will see more concrete examples of possible models of the O-I beyond what is known currently.

Ultimately the use of the O-I in games is one that can now be permitted, while remaining controversial in wargames that strive to be more historically accurate, as it was only a prototype of which currently no photo evidence exists (there are claims that a piece of the O-I tank treads exists in recent color photos).

The more “realistic” choice for a very large tank (albeit with smaller guns) would be the versatile and monstrous amphibious Ka-Chi, which as of this writing has yet to make it into many, if any games, despite 19 having been built and field tested with photo evidence of their deployment for testing beyond the typical proving grounds / garage that other prototypes like the Chi-To were limited to.

Still, for fans of fantasy tank match-ups in games, the O-I tank project(s) offer something exciting for Japan, with very large guns and multiple turrets. A last ditch defense of mainland Japan with a handful of O-I tanks in an alternate past where the atomic bombs were not dropped could make for some exciting gaming for either side of the conflict.